The present invention relates to a (meth)acrylic resin composition, in which thermal degradation when processing is inhibited and discoloring hardly occurs, and a capstock and extrusion-molded article comprising the (meth)acrylic resin composition.
(Meth)acrylic resin has conventionally been used as substitute material for glass due to water-clear appearance characteristic of (meth)acrylic resin. Utilizing its excellent weather resistance, (meth)acrylic resin is also used as a capstock, which covers the surface of materials used outdoors.
(Meth)acrylic resin is prepared by radical polymerization such as bulk polymerization, suspension polymerization and emulsion polymerization and of these, bulk polymerization is most widely employed, in that the amount of impurities is small. In the case of bulk polymerization, the final form of the (meth)acrylic resin product is pellets. However, because most of the ingredients added when forming the composition such as an impact modifier and a processing aid are in the form of powder, the pellets and the powder separate when compounding, transporting and extrusion molding and mixing and dispersion of the constituent elements may become inhomogeneous.
In contrast, in the case of suspension polymerization, particles having a size of a few dozen to a few thousand microns can be obtained by polymerization and after going through steps of washing and drying, a (meth)acrylic resin product can be obtained as powder of true spherical shape. Therefore, there is the advantage that mixing and dispersion with the other powder form ingredients are excellent. However, in order to maintain the dispersion stability of monomer and polymer drops in polymerization, a large amount of a dispersion stabilizer or a dispersion stabilizing aid must be added as a secondary ingredient. As a result, a small amount of impurities remain even after washing and this is considered to be the source of degradation when processing. In the case that the amount of the dispersion stabilizer is decreased, because the dispersion stabilization of monomer and polymer drops decrease along with decrease in concentration, the monomer and polymer drops adhere together and consequently, polymer particles with a wide particle distribution are produced. Also, when the balance of conditions is lost, the entire polymerization system may agglomerate. Therefore, the problem of thermal degradation when processing makes selecting the type of dispersion stabilizer and adjusting the concentration difficult, in terms of stable production and particle size control.
With respect to improving thermal stability of (meth)acrylic resin, disclosed are the method of adding an inorganic phosphorus-type reducing agent (U.S. Pat. No. 5,063,259), the method of improving coloring by compounding an acrylic rubber core-shell polymer, in which an organic or inorganic phosphorus-type reducing agent is added (JP-A-4-226558) and the method of improving coloring by compounding MBS containing hindered phenol and phosphite (JP-A-7-286087). These methods are effective with respect to thermal degradation when processing of (meth)acrylic resin, but cannot inhibit coloring due to thermal degradation caused by the dispersion stabilizer used for the (meth)acrylic resin in suspension polymerization of the present invention.
In this way, in reality, a (meth)acrylic resin composition, which is in the form of powder and in which thermal degradation when processing is small, does not exist.